1. The problem statement, all variables and given/known data The system is in equilibrium. All the surfaces in the image are rough. The direction of friction on B due to A is : (a) is upward (b) is downward (c) is zero (d) depends on the masses of A and B. ANS : (a) 2. Relevant equations MAg = 2μN (Friction due to wall and due to B) N=F 3. The attempt at a solution Since the gravitational force is balanced by the frictional force on A by B and the wall, the direction of frictional force due to B and the wall on A must be upward and therefore, direction of force due to A on B must be downward, i.e., (b), but I am not sure that my reasoning is correct and I don't understand why it should be upward?